Throughout this application, various publications, patents, and pending patent applications are referred to by an identifying citation. The disclosures of the publications and patents referenced in this application are hereby incorporated by reference into the present disclosure to more fully describe the state of the art to which this invention pertains.
Thermal runaway and other heat-related safety problems with lithium-ion and other lithium-based batteries are well known. Because of the relatively low initiation temperature and rapid exothermic nature of thermal runaway, a heat-activated shutdown of the lithium battery is needed at a temperature well below the melting point of lithium at 179° C. and the rapid vaporization of the organic electrolyte solvents around this same temperature range. A rapid thermal shutdown at a temperature of 135° C. or lower, preferably in the range of 100° C. to 110° C., is highly desirable. As lithium-based batteries become more energetic and are increasingly utilized for high power applications such as hybrid electric vehicles (HEVs), the need for rapid thermal shutdown of the cells increases.
A variety of tests are used to evaluate the safety of batteries against heat buildup. These tests include, for example, heating the battery cell in an oven for 30 minutes at 150° C., overcharging the cell, and internal short-circuiting of the cell.
One approach to thermal shutdown of lithium-based batteries is adding a positive thermal coefficient (PTC) device to the circuitry of the lithium-based cell. The PTC device increases its resistivity when the temperature increases. This increased resistivity increases the internal resistance of the cell and causes the current producing operation of the cell to greatly diminish or to stop. This provides a level of safety although the PTC device is relatively expensive and adds weight, volume, and complexity to the design of the cell.
Another approach to thermal shutdown of lithium-based batteries is a multi-layer porous polyolefin separator approach where an inner layer of a low-melting plastic, such as polyethylene, has an outer layer of a high-melting plastic, such as polypropylene, on both sides. The low-melting porous plastic layer melts when the temperature of the cell exceeds its melting point and becomes non-porous. This increases the internal resistance of the cell by interfering with the diffusion of lithium ions and sharply lowering the conductivity of the electrolyte between the two electrodes of the cell. This provides a level of safety although the relatively high melting point of polyethylene at about 135° C. is significantly higher than the preferred shutdown range of 100° C. to 110° C. and the rate of its melting may not be fast enough for effective shutdown in some cases.